1. Field of the Invention
The present invention relates to a current reference circuit and more particularly, to a current reference circuit capable of generating a reference current with substantially no temperature dependence, which is able to be operated at a supply voltage of approximately 1 V.
2. Description of the Prior Art
Conventionally, a current reference circuit is used for generating a reference current having a constant current value and no temperature dependence. An example of conventional current reference circuits is shown in FIG. 1, which was disclosed in Proceedings of the 1994 IEICE spring conference, No. C-663.
In FIG. 1, three diodes D101, D102, and D103 are cascode-connected and driven by a constant current I.sub.0 flowing therethrough. A base of an npn bipolar transistor Q117 is connected to an anode of the diode D101. A cathode of the diode D103 is connected to the ground. An emitter of the transistor Q117 is connected to one end of a resistor R105. A collector of the transistor Q117 is applied with a supply voltage V.sub.DD.
An npn bipolar transistor Q114 has a base and a collector coupled together. An npn-bipolar transistor Q115 has a base connected to the base of the transistor Q114. Emitters of the transistors Q114 and Q115 are connected to the ground. These two transistors Q114 and Q115 constitute a simple current mirror subcircuit.
An npn bipolar transistor Q116 has a base connected to the bases of the transistors Q114 and Q115, and a collector connected to a collector of the transistor Q115. The transistor Q116 has an emitter connected to one end of an emitter resistor R106. The other end of the resistor R106 is connected to the ground. Thus, the emitter of the transistor Q116 is connected to the ground through the emitter resistor R106. These two transistors Q115 and Q116 and the emitter resistor R106 constitute a Widlar current mirror subcircuit.
When a current flowing through the resistor R105 is defined as I.sub.1, a mirror current I.sub.2 of the simple current mirror subcircuit with respect to the current I.sub.1 flows at the collector of the transistor Q115, where I.sub.2 =I.sub.1. Also, a mirror current I.sub.3 of the Widlar current mirror subcircuit with respect to the current I.sub.1 flows at the collector of the transistor Q116. I.sub.3 is expressed as I.sub.3 =(V.sub.BE114 -V.sub.BE116)/R106, where V.sub.BE114 is the base-to-emitter voltage of the transistor Q114, V.sub.BE116 is the base-to-emitter voltage of the transistor Q116, and R.sub.106 is the resistance of the emitter resistor R106.
A p-channel Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) M111 has a gate and a drain coupled together to be connected to the coupled collectors of the bipolar transistors Q115 and Q116. The MOSFET M111 has a source applied with a supply voltage V.sub.DD. A p-channel MOSFET M112 has a gate connected to the gate of the MOSFET M111 and a source applied with the supply voltage V.sub.DD. The MOSFET M112 has a drain from which a reference current I.sub.REF with no temperature dependence is derived. These two MOSFETs M111 and M112 constitute a simple current mirror subcircuit.
A p-channel MOSFET M110 has a gate connected to the coupled gates of the MOSFETs M111 and M112. The MOSFET M110 has a source applied with the supply voltage V.sub.DD and a drain connected to the connection point of the base of the transistor Q117 and the anode of the diode D101. The two MOSFETs M111 and M110 constitute a simple current mirror subcircuit.
The collectors of the transistors Q115 and Q116 are coupled together and are connected to the coupled gate and drain of the MOSFET M111. Therefore, the sum of the mirror currents I.sub.2 and I.sub.3, i.e., (I.sub.2 +I.sub.3), flows through the MOSFET M111. Since the MOSFETs M111 and M112 constitute the simple current mirror subcircuit, a mirror current of the sum current (I.sub.2 +I.sub.3) flows through the MOSFET M112 as the reference current I.sub.REF, which is derived from the drain of the MOSFET M112. Therefore, the reference current I.sub.REF is expressed as I.sub.REF =I.sub.2 +I.sub.3.
At the same time, since the MOSFETs M111 and M110 also constitute the simple current mirror subcircuit, another mirror current of the sum current (I.sub.2 +I.sub.3) flows through the MOSFET M110 and the cascode-connected diodes D101, D102, and D103 as the driving current I.sub.0. The current I.sub.0 flowing through the diodes D101, D102, and D103 generates a voltage drop, which generates a bias current for the transistor Q117. This bias current is supplied to the base of the transistor Q117 and makes it possible to form a current loop extending along the bipolar transistors Q117, Q114, Q115 and Q116, the resistor R105, and the MOSFETs M111 and M110.
One end of a capacitor C101 is connected to the connection point of the base of the transistor Q117, the drain of the MOSFET M110, and the anode of the diode D101. The other end of the capacitor C101 is connected to the ground. A voltage V.sub.SS is applied to the capacitor C101 to thereby store an electric charge therein. The stored charge in the capacitor C101 is discharged at the start of operation to thereby generate a start-up current, which is supplied to the base of the transistor Q117. Thus, the conventional current reference circuit of FIG. 1 starts to operate.
With the conventional current reference circuit in FIG. 1, supposing that the bias current generated by the diodes D101, D102, and D103 has no temperature dependence, the base-to-emitter voltage V.sub.BE114 of the transistor Q114 has the same negative temperature coefficient of -2 mV/deg as that of the forward voltage drop of the diodes D101, D102, and D103. Therefore, the current I.sub.1, which flows through the resistor R105, has a negative temperature coefficient on the supposition that the temperature coefficient of the resistor R105 is sufficiently small.
Since the current I.sub.1 further flows through the diode-connected transistor Q114, the collector current (i.e., the mirror current) I.sub.2 of the transistor Q115 also has a negative temperature coefficient.
On the other hand, the mirror current I.sub.3 of the Widlar current mirror subcircuit has a positive temperature coefficient. In other words, the Widlar current mirror subcircuit generates the mirror current I.sub.3 while it converts the negative temperature coefficient of the current I.sub.1 into a positive one.
The output current I.sub.2 of the simple current mirror subcircuit formed by the transistors Q114 and Q115 and the output current I.sub.3 of the Widlar current mirror subcircuit formed by the transistors Q114 and Q116 and the resistor R106 are added to be supplied to the drain of the MOSFET M111. Therefore, by suitably weighting the respective mirror currents I.sub.2 and I.sub.3, in other words, by suitably setting the adding ratio of the currents I.sub.2 and I.sub.3, the temperature coefficient of the sum current (I.sub.2 +I.sub.3) supplied to the drain of the MOSFET M111 can be set as zero.
Thus, the temperature dependence of the reference current I.sub.REF (=I.sub.2 +I.sub.3) can be removed.
With the conventional current reference circuit in FIG. 1, however, the three cascade-connected diodes D101, D102, and D103 are necessary to drive the simple current mirror subcircuit formed by the transistors Q114 and Q115 and the Widlar current mirror subcircuit formed by the transistors Q114 and Q116 and the resistor R106. Because each of the diodes D101, D102, and D103 requires a power supply voltage of at least 0.7 V, a necessary power supply voltage for the cascade-connected diodes D101, D102, and D103 is at least 2.1 V.
This means that the conventional current reference circuit of FIG. 1 is unable to be operated by a single battery having a supply voltage of 1.2 V, which disturbs the miniaturization of apparatuses using this current reference circuit.